Washover is defined as the condition of occasional (not “frequent,” continuous or permanent) partial or complete inundation and exposure of a top surface of a floating object to a fluid. Typically, at sea washover is caused by wave action, wind action, the dynamics of towing a floating object (body) or a combination of the interaction of these factors. Washover may adversely affect performance of floating objects equipped with electronic devices such as antennas, photovoltaic arrays, instrumentation, radio, and auditory or visual beacons. For a low-profile towed body, washover depth is usually within the range of less than a centimeter (0.4″) to 30 cm (12″). Typically, an inundation lasts less than one second.
Fiber optic-based methods of the present invention provide three dimensional (3D) spatial and temporal detection, measurement and visualization of washover in both freshwater and seawater and may be applied to other transparent or translucent fluids. Temporal and spatial optical-based washover measurements provide an empirically-based benchmarking process for assessing the computational fluid dynamics of fluid flow around an arbitrarily shaped surface of an object.
A preferred embodiment of the present invention permits simultaneous measurement of temporal and spatial washover dynamics that are non-invasive both physically and electro-magnetically. For example, measurements may be made while taking measurements of water wave/electromagnetic (RF) interaction of onboard electronics, e.g., low profile antennas. Data may be acquired and displayed in real-time, as well as stored for post-processing, analysis, playback and display.
The ability to capture key parameters of washover as data, as well as to visualize washover events in real-time, enables designers to improve hydrostatic and hydrodynamic profiles of these objects. Currently, there are no purpose-built non-interfering devices, systems, or methods that provide this information. Thus, there exists a need for an optical washover measurement system that permits real-time acquisition, three-dimensional mapping, and visualization of seawater washover dynamics without interfering with the operation of onboard electronics. The real-time data acquired with this system may be used to analyze hydrodynamics and electromagnetic interaction with seawater while an electromagnetic device is operating on the object. It is also useful for use in conjunction with computer simulations and modeling to optimize hardware design “off the bench.”
A preferred embodiment of the present invention was developed to meet the need to temporally and spatially quantify seawater wave interactions in the VHF through microwave frequency range while electromagnetic energy emanates from a low profile towed body. At these RF frequencies, the skin depth (a measure of the maximum depth to which electromagnetic energy penetrates) is on the order of one centimeter (0.4″).
Skin depth, δs (m), is defined as the depth at which a conductor's current is reduced to 0.368 of the surface value, the equivalent to a power loss of 8.7 dB. Doubling the value of δs doubles the loss. Skin depth is frequency dependent, i.e., the higher the frequency, the shallower the skin depth. Skin depth can be defined mathematically as:                               δ          s                =                                            2                              ω                ⁢                                                                   ⁢                μ                ⁢                                                                   ⁢                σ                                              =                                    ρ                              π                ⁢                                                                   ⁢                f                ⁢                                                                   ⁢                μ                                                                        (        1        )            
where:                μ=permeability (for seawater, μ=4π×10−7 Henrys/m)        ρ=resistivity (Ω−m)        ω=radian frequency=2πf (Hz)        σ=conductivity (mho/m)=ρ−1, (where mho [Ω−1]=Siemen [S])        
FIG. 1 illustrates the frequency dependency of skin depth for two values of resistivity that may be expected for seawater. Across the VHF and UHF spectrum, skin depth is on the order of a couple of centimeters (0.5-0.75″) or less and decreases with increasing frequency. For a frequency of 1000 MHz (1 GHz) the skin depth is approximately 0.75 cm (0.3″). Thus, at the preferred higher frequencies even relatively shallow washover has a profound effect on transmitted and received signal levels.
Currently, there are no existing devices, systems, or methods that can provide this information in a non-interfering manner when washover patterns must be studied simultaneously with the evaluation of an onboard electromagnetic antenna or other electromagnetic device. A technique using electrically powered electrodes makes appropriate washover measurements but not on a non-interference basis. U.S. patent application Ser. No. 10/318297, filed Dec. 13, 2002, by Yankielun and Clark. This technique relies on an extensive network of metallic wired electrodes, which interfere with electromagnetic radiation patterns when applied in the immediate proximity of an antenna under test. Accordingly, it may not be used to perform non-interfering measurements simultaneously with operation of onboard electronics.
A preferred embodiment of the present invention uses opto-electronic techniques to provide a method and apparatus that facilitates real-time acquisition of pertinent parameters to enable two or three-dimensional mapping and visualization of washover. In a specific application, it provides an empirically based benchmarking process for computational fluid dynamic assessments of turbulent flow around arbitrarily shaped surface-towed bodies.